It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470 and 4,550,069. These include alkali-soluble novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. Development contrast refers to a comparison between the percentage of film loss in the exposed area of development with the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the expoeed area is substantially completely dissolved away and thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely.
Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
In order to achieve the desired sensitivity and contrast, it is necessary to have a sufficiently high concentration of photosensitive compounds in the photoresist to prevent dissolution of the unexposed resist during development. High concentrations photosensitizer may be achieved by dissolving a mixture of highly soluble and less soluble photosensitizers in the photoresist solution or by supersaturating the photoresist solution with a photosensitizer.
However, solution concentration of photosensitizer near saturation can lead to a short shelf life due to solution instability and precipitation of the sensitizer prior to or during the use of the product. This phenomenon is referred to in U.S. Pat. Nos. 4,397,937 and 4,526,856, both of which are hereby incorporated by reference.
Clecak et al. in U.S. Pat. No. 4,397,937 discloses improved solubility by using as a sensitizer a bisester of 1-oxo-2-diazonaphthalene sulfonic acid and an unsymmetrical primary or secondary aliphatic diol which is a mixture of geometric and diastereoisomers. Esterification of the aliphatic diol at one end with a diazonaphthoquinone molecule having the acid group in the 5 position and at the other end with a diazonophthoquinone molecule having the sulfonyl group in the 4 position is disclosed. However, the patent emphasizes the importance of limiting the invention to unsymmetrical aliphatic diols. Clecak et al. does not disclose mixed esters of o-quinone diazide acids and non-light-sensitive organic acids.
According to Lewis et al. in U.S. Pat. No. 4,526,856, the problem of obtaining higher concentrations of sensitizer was addressed by modifying the solvent system of the photoresist formulations. The solvent composition which consist of cyclopentanone, or cyclopentanone and cyclohexanone with an aliphatic alcohol, when used in certain critical ratios provides good solubility.
In contrast, the present invention provides photosensitizers which are the condensation products of 1,2 naphthoquinonediazide-5-sulfonic acid and organic acids with aromatic diols and polyols. The photosensitizer compositions of the invention exhibit excellent solubility and resistance to precipitation when formulated in photoresist compositions. Also they may be dissolved into photoresist compositions at higher solution concentrations than previously known comparable photosensitizers. The photoresist compositions of the invention exhibit excellent solution stability and an improved shelf life. At the same time they also exhibit excellent photosensitivity and contrast properties.